Silicone Wax Modified Low Wear Polyoxymethylene

ABSTRACT

A tribologically modified polyoxymethylene polymer composition is comprised of a polyoxymethylene polymer and at least one tribological modifier. The tribological modifier may comprise at least one silicone wax having a molecular weight of less than 40,000 g/mol. The composition may exhibit a dynamic coefficient of friction against a counter-material of from about 0.1 to about 0.5. The composition provides polymer articles with improved tribological properties.

RELATED APPLICATIONS

The present application claims priority to and is based on U.S. Provisional Patent Application Ser. No. 61/919,929, filed on Dec. 23, 2013, and which is hereby incorporated by reference.

BACKGROUND

Polyacetal polymers, which are commonly referred to as polyoxymethylene polymers, have become established as exceptionally useful engineering materials in a variety of applications. For instance, because polyoxymethylene polymers have excellent mechanical properties, fatigue resistance, abrasion resistance, chemical resistance, and moldability, they are widely used in constructing polymer articles, such as articles for use in the automotive industry and the electrical industry.

The mechanical properties of polyoxymethylene molding compositions are the reason for their use in numerous applications. To improve their properties, polyoxymethylene polymers are often provided with additives to adapt the properties for a specific application, for example by using reinforcing fibers or tribological modifiers. For instance, polyoxymethylene polymers have been combined with a tribological modifier for producing polymer compositions well suited for use in tribological applications where a polymer article comprising the composition is in moving contact with other articles, such as metal articles, plastic articles, and the like. As such, the addition of a tribological modifier can provide a composition with improved tribological properties such as a reduced coefficient of friction, reduced frictional noise, and low wear.

In the past, high molecular weight polyolefins have been used to improve the wear resistance of polyoxymethylene resins. For instance, U.S. Pat. No. 5,482,987, which is incorporated herein by reference in its entirety, discloses a self-lubricating, low wear composition containing a polyoxymethylene and a lubricating system comprising a high molecular weight polyethylene, a high density polyethylene, and other components. U.S. Pat. No. 5,641,824, which is incorporated herein by reference in its entirety, discloses a self-lubricating melt blend of a polyoxymethylene and an ultra-high molecular weight polyethylene. However, polyoxymethylene compositions modified with these high molecular weight polyethylenes may have a less than desirable surface appearance as well as defects that may detract from the wear properties of the compositions and articles produced therefrom.

Although polyoxymethylene has been tribologically modified in the past, further improvements are still necessary. For instance, a need exists for providing a polyoxymethylene polymer composition and a polymer article produced therefrom with improved tribological properties. In particular, a need exists for providing a composition and a polymer article produced therefrom with improved wear properties and a reduced coefficient of friction when in contact with other moving articles.

SUMMARY

According to one embodiment, the present disclosure is directed to a polyoxymethylene polymer composition. The composition is comprised of a polyoxymethylene polymer, such as a polyoxymethylene copolymer, and at least one silicone wax having a molecular weight of less than 40,000 g/mol, such as less than about 30,000 g/mol. The polyoxymethylene polymer may be present in the composition in an amount of at least about 80 wt. %. The silicone wax may be present in the composition in an amount of from about 0.1 to about 10 wt. %.

The silicone wax may be comprised of a modified silicone wax such as a modified polydimethylsiloxane wax. The silicone wax, such as the modified silicone wax, may be comprised of an alkyl group having more than 2 carbon atoms, an amine group, a polyether group, a hydroxyl group, or any combination thereof. For instance, the alkyl group may be comprised of from 2 to 40 carbon atoms. The polyether group may be comprised of from 2 to 10 carbon atoms. The hydroxyl modified silicone wax may have a hydroxyl value of from about 5 to about 150.

The composition may further comprise a nucleating agent in an amount of from about 0.05 wt. % to about 0.5 wt. %. The nucleating agent may be a polyoxymethylene terpolymer. The composition may further comprise an antioxidant in an amount of from about 0.05 wt. % to about 0.5 wt. %. The composition may further comprise a formaldehyde scavenger in an amount of from about 0.005 wt. % to about 0.1 wt. %. The composition may further comprise an acid scavenger in an amount of from about 0.005 wt. % to about 0.1 wt. %. The composition may further comprise a lubricant in an amount of from about 0.05 wt. % to about 0.5 wt %.

According to the present disclosure, the silicone wax provides a composition with improved tribological properties such as a reduced coefficient of friction and reduced wear in comparison to an unmodified polyoxymethylene polymer composition, such as a tribologically unmodified composition. For instance, the composition may exhibit a dynamic coefficient of friction against a counter-material of from about 0.1 to about 0.5, such as from about 0.2 to about 0.3. For instance, the counter-material may be a polyoxymethylene surface, such as an tribologically unmodified polyoxymethylene surface.

Other features and aspects of the present disclosure are discussed in greater detail below.

DETAILED DESCRIPTION

Reference now will be made in detail to the embodiments of the invention, one or more examples of which are set forth below. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment, can be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention cover such modifications and variations.

In general, the present disclosure is directed to a polyoxymethylene polymer composition and a polymer article comprising the composition. In general, the composition comprises a polyoxymethylene polymer and at least one tribological modifier. For instance, the tribological modifier may include a silicone wax, such as a modified silicone wax.

The tribological modifier can provide a polyoxymethylene polymer composition and a polymer article produced therefrom with improved tribological properties. These properties may include improved sliding properties, improved wear properties, reduced frictional noise, and a reduced coefficient of friction when contacting the composition and/or polymer article against other surfaces. In particular, the composition and polymer article made from the composition may exhibit improved tribological properties when contacted against other surfaces or counter-materials while still exhibiting desirable mechanical properties.

Polyoxymethylene Polymer

According to the present disclosure, the polyoxymethylene polymer composition and polymer article produced therefrom are comprised of at least one polyoxymethylene polymer.

The preparation of the polyoxymethylene polymer can be carried out by polymerization of polyoxymethylene-forming monomers, such as trioxane or a mixture of trioxane and a cyclic acetal such as dioxolane in the presence of ethylene glycol as a molecular weight regulator. The polyoxymethylene polymer used in the polymer composition may comprise a homopolymer or a copolymer. According to one embodiment, the polyoxymethylene is a homo- or copolymer which comprises at least 50 mol. %, such as at least 75 mol. %, such as at least 90 mol. %, such as at least 95 mol. %, and such as even at least 97 mol. % of —CH₂O— repeat units.

In one embodiment, the polyoxymethylene polymer is a polyoxymethylene copolymer. The copolymer can contain from about 0.1 mol. % to about 20 mol. %, such as from about 0.5 mol. % to about 10 mol. %, such as from about 1.0 to about 5 mol. % of repeat units that comprise a saturated or ethylenically unsaturated alkylene group having at least 2 carbon atoms, or a cycloalkylene group, which has sulfur atoms or oxygen atoms in the chain and may include one or more substituents selected from the group consisting of alkyl cycloalkyl, aryl, aralkyl, heteroaryl, halogen or alkoxy. In one embodiment, a cyclic ether or acetal is used that can be introduced into the copolymer via a ring-opening reaction.

Preferred cyclic ethers or acetals are those of the formula:

in which x is 0 or 1 and R² is a C₂-C₄-alkylene group which, if appropriate, has one or more substituents which are C₁-C₄-akyl groups, or are C₁-C₄-alkoxy groups, and/or are halogen atoms, preferably chlorine atoms. Merely by way of example, mention may be made of ethylene oxide, propylene 1,2-oxide, butylene 1,2-oxide, butylene 1,3-oxide, 1,3-dioxane, 1,3-dioxolane, and 1,3-dioxepan as cyclic ethers, and also of linear oligo- or polyformals, such as polydioxolane or polydioxepan, as comonomers.

In one embodiment, the polyoxymethylene copolymer is composed of from 99.5 to 95 mol. % of trioxane and of from 0.5 to 5 mol. %, such as from 0.5 to 4 mol. % of one of the above-mentioned comonomers.

The polymerization can be effected as precipitation polymerization or in the melt. By a suitable choice of the polymerization parameters, such as duration of polymerization or amount of molecular weight regulator, the molecular weight and hence the MVR value of the resulting polymer can be adjusted.

In one embodiment, a polyoxymethylene polymer with hydroxyl terminal groups can be produced using a cationic polymerization process followed by solution hydrolysis to remove any unstable end groups. During cationic polymerization, a glycol, such as ethylene glycol can be used as a chain terminating agent. The cationic polymerization results in a bimodal molecular weight distribution containing low molecular weight constituents. In one particular embodiment, the low molecular weight constituents can be significantly reduced by conducting the polymerization using a heteropoly acid such as phosphotungstic acid as the catalyst. When using a heteropoly acid as the catalyst, for instance, the amount of low molecular weight constituents can be less than about 2 wt. %.

A heteropoly acid refers to polyacids formed by the condensation of different kinds of oxo acids through dehydration and contains a mono- or poly-nuclear complex ion wherein a hetero element is present in the center and the oxo acid residues are condensed through oxygen atoms. Such a heteropoly acid is represented by the formula:

H_(x)[MmM′nOz]_(y)H₂O

wherein

M represents an element selected from the group consisting of P, Si, Ge, Sn, As, Sb, U, Mn, Re, Cu, Ni, Ti, Co, Fe, Cr, Th or Ce,

M′ represents an element selected from the group consisting of W, Mo, V or Nb,

m is 1 to 10,

n is 6 to 40,

z is 10 to 100,

x is an integer of 1 or above, and

y is 0 to 50.

The central element (M) in the formula described above may be composed of one or more kinds of elements selected from P and Si and the coordinate element (M′) is composed of at least one element selected from W, Mo and V, particularly W or Mo.

Specific examples of heteropoly acids are phosphomolybdic acid, phosphotungstic acid, phosphomolybdotungstic acid, phosphomolybdovanadic acid, phosphomolybdotungstovanadic acid, phosphotungstovanadic acid, silicotungstic acid, silicomolybdic acid, silicomolybdotungstic acid, silicomolybdotungstovanadic acid and acid salts thereof. Excellent results have been achieved with heteropoly acids selected from 12-molybdophosphoric acid (H₃PMo₁₂O₄₀) and 12-tungstophosphoric acid (H₃PW₁₂O₄₀) and mixtures thereof.

The heteropoly acid may be dissolved in an alkyl ester of a polybasic carboxylic acid. It has been found that alkyl esters of polybasic carboxylic acid are effective to dissolve the heteropoly acids or salts thereof at room temperature (25° C.).

The alkyl ester of the polybasic carboxylic acid can easily be separated from the production stream since no azeotropic mixtures are formed. Additionally, the alkyl ester of the polybasic carboxylic acid used to dissolve the heteropoly acid or an acid salt thereof fulfills the safety aspects and environmental aspects and, moreover, is inert under the conditions for the manufacturing of oxymethylene polymers.

Preferably the alkyl ester of a polybasic carboxylic acid is an alkyl ester of an aliphatic dicarboxylic acid of the formula:

(ROOC)—(CH₂)n-(COOR′)

wherein

n is an integer from 2 to 12, preferably 3 to 6 and

R and R′ represent independently from each other an alkyl group having 1 to 4 carbon atoms, preferably selected from the group consisting of methyl, ethyl, n-propyl, iso-propyl, n-butyl, iso-butyl and tert-butyl.

In one embodiment, the polybasic carboxylic acid comprises the dimethyl or diethyl ester of the above-mentioned formula, such as a dimethyl adipate (DMA).

The alkyl ester of the polybasic carboxylic acid may also be represented by the following formula:

(ROOC)₂—CH—(CH₂)m-CH—(COOR′)₂

wherein

m is an integer from 0 to 10, preferably from 2 to 4 and

R and R′ are independently from each other alkyl groups having 1 to 4 carbon atoms, preferably selected from the group consisting of methyl, ethyl, n-propyl, iso-propyl, n-butyl, iso-butyl and tert-butyl.

Particularly preferred components which can be used to dissolve the heteropoly acid according to the above formula are butanetetracarboxylic acid tetratethyl ester or butantetracarboxylic acid tetramethyl ester.

Specific examples of the alkyl ester of a polybasic carboxylic acid are dimethyl glutaric acid, dimethyl adipic acid, dimethyl pimelic acid, dimethyl suberic acid, diethyl glutaric acid, diethyl adipic acid, diethyl pimelic acid, diethyl suberic acid, diemethyl phthalic acid, dimethyl isophthalic acid, dimethyl terephthalic acid, diethyl phthalic acid, diethyl isophthalic acid, diethyl terephthalic acid, butantetracarboxylic acid tetramethylester and butantetracarboxylic acid tetraethylester as well as mixtures thereof. Other examples include dimethylisophthalate, diethylisophthalate, dimethylterephthalate or diethylterephthalate.

Preferably, the heteropoly acid is dissolved in the alkyl ester of the polybasic carboxylic acid in an amount lower than 5 wt. %, preferably in an amount ranging from 0.01 to 5 wt. %, wherein the weight is based on the entire solution.

The polyoxymethylene polymer of the present disclosure may be a linear or unbranched polymer or copolymer or a branched polymer. In one particular embodiment, the polyoxymethylene copolymer is a linear or unbranched copolymer. For instance, the polyoxymethylene polymer may be comprised primarily of polyoxymethylene homopolymers or copolymers such as linear or unbranched polyoxymethylene homopolymers or copolymers. For instance, in one embodiment, the linear or unbranched polyoxymethylene homopolymer or copolymer is present in the composition in an amount of at least about 60 wt. %, such as at least 70 wt. %, such as at least 80 wt. %, such as at least 85 wt. %, such as at least 90 wt. %, such as at least 95 wt. %. In one embodiment, a branched polyoxymethylene polymer may be present in the composition in an amount of less than about 50 wt. %, such as less than about 25 wt. %, such as less than about 10 wt. %, such as less than about 5 wt. %, such as less than about 2.5 wt. %, such as less than about 1 wt. %.

In some embodiments, the polymer composition of the present disclosure may contain other polyoxymethylene homopolymers and/or polyoxymethylene copolymers. Such polymers, for instance, may generally be unbranched linear polymers which contain at least 80%, such as at least 90%, oxymethylene units.

The polyoxymethylene polymer can have any suitable molecular weight. The molecular weight of the polymer, for instance, can be from about 4,000 grams per mol to about 20,000 g/mol. In other embodiments, however, the molecular weight can be well above 20,000 g/mol, such as from about 20,000 g/mol to about 100,000 g/mol.

The polyoxymethylene polymer present in the composition can generally have a melt flow index (MFI) ranging from about 1 to about 50 g/10 min, as determined according to ISO 1133 at 190° C. and 2.16 kg, though polyoxymethylenes having a higher or lower melt flow index are also encompassed herein. For example, the polyoxymethylene polymer may be a low or mid-molecular weight polyoxymethylene that has a melt flow index of greater than about 5 g/10 min, greater than about 10 g/10 min, or greater than about 15 g/10 min. The melt flow index of the polyoxymethylene polymer can be less than about 25 g/10 min, less than about 20 g/10 min, less than about 18 g/10 min, less than about 15 g/10 min, less than about 13 g/10 min, or less than about 12 g/10 min. The polyoxymethylene polymer may for instance be a high molecular weight polyoxymethylene that has a melt flow index of less than about 5 g/10 min, less than about 3 g/10 min, or less than about 2 g/10 min.

The polyoxymethylene polymer may contain a relatively high amount of functional groups, such as hydroxyl groups in the terminal positions. More particularly, the polyoxymethylene polymer can have terminal hydroxyl groups, for example hydroxyethylene groups and/or hydroxyl side groups, in at least more than about 50% of all the terminal sites on the polymer. It should be understood that the total number of terminal groups present includes all side terminal groups. In addition to the terminal hydroxyl groups, the polyoxymethylene polymer may also have other terminal groups usual for these polymers such as alkoxy groups, formate groups, acetate groups or hemiacetal groups.

The polyoxymethylene polymer may also optionally have a relatively low amount of low molecular weight constituents. As used herein, low molecular weight constituents (or fractions) refer to constituents having molecular weights below 10,000 dalton. In this regard, the polyoxymethylene polymer can contain low molecular weight constituents in an amount less than about 10 wt. %, based on the total weight of the polyoxymethylene. In certain embodiments, for instance, the polyoxymethylene polymer may contain low molecular weight constituents in an amount less than about 5 wt. %, such as in an amount less than about 3 wt. %, such as even in an amount less than about 2 wt. %.

The polyoxymethylene polymer may be present in the composition in an amount of at least 60 wt. %, such as at least 70 wt. %, such as at least 80 wt. %, such as at least 85 wt. %, such as at least 90 wt. %, such as at least 95 wt. %. In general, the polyoxymethylene polymer is present in an amount of less than about 100 wt. %, such as less than about 99 wt. %, such as less than about 98 wt. %, wherein the weight is based on the total weight of the composition.

Tribological Modifiers

According to the present disclosure, the polyoxymethylene polymer composition and polymer article produced therefrom are comprised of at least one tribological modifier. In general, a tribological modifier, as is generally known in the art, may be a component or modifier that impacts the interaction between two surfaces.

In one embodiment, the tribological modifier may include a silicone wax such as a polysiloxane wax or polyorganosiloxane wax. For instance, these waxes may include polydialkylsiloxanes (e.g., polydimethylsiloxane), polyalkylarylsiloxanes (e.g., polyphenylmethysiloxane), and the like. The polysiloxane may be present in the form of a homopolymer, copolymer, and/or oligomer having the properties identified herein.

In addition, the silicone wax, such as the polysiloxane or polyorganosiloxane, may be modified with a substituent group to provide a modified silicone wax containing the substituent group or functional group. As an example, one or more methyl groups of a polydimethylsiloxane may be substituted or modified to provide a substituent group. However, it should be understood that this modification can occur with other polydialkylsiloxanes or polyalkylarylsiloxanes known in the art.

The modified silicone wax may contain an alkyl group having 2 or more carbon atoms, an epoxy group, a hydroxyl group, a hydroxyalkyl group, a carboxyl group, an ester group, an amino group or a substituted amino group (e.g., alkyl amino group, dialkyl amino group), a halogenated group such as a halogenated alkyl group and/or a halogenated phenyl group (e.g., group comprising a fluorine), a vinyl group, an alkoxy group, an ether group or a polyether group, an acrylic group, an isocyanate group, a hydride, and/or a quaternary compound such as a dialkyl quaternary compound, fatty amino quaternary compound, polyether fatty quaternary compound.

The modification may occur in the main chain of the silicone wax such that the modification occurs to the side/pendant groups or the modification may occur at the terminal groups of the silicone wax.

The substituent groups may be used singly or in combination to produce a modified silicone wax. For instance, the modified silicone wax may contain one substituent group or more than one substituent group. For instance, the modified silicone wax may be modified to include only an alkyl group having 2 or more carbon atoms, only an amino group, only a polyether group, only a hydroxyl group or hydroxyalkyl group, an alkyl group having 2 or more carbon atoms in combination with an amino group, an amino group in combination with a polyether group, a polyether group in combination with a hydroxyl group or hydroxyalkyl group, an amino group with a hydroxyl group or hydroxyalkyl group, an alkyl group having 2 or more carbon atoms in combination with a hydroxyl group or hydroxyalkyl group, and the like. Any combination of substituent groups identified above maybe present in the silicone wax, such as the modified silicone wax.

In one embodiment, the functional groups may be reactive functional groups. For instance, the reactive groups may react with one or more of the components in the composition. For instance, reactive functional groups may include, but are not limited to, a hydroxyl group, an amino group, an epoxy group, and the like. In another embodiment, the functional groups may be non-reactive functional groups. For instance, non-reactive functional groups may include, but are not limited to, an alkyl group and the like.

In one embodiment, the silicone wax may be a hydroxyl modified silicone wax such that the silicone wax has hydroxyl functionality. For instance, the hydroxyl functionality may be present in the side/pendant groups, terminal groups, or a combination thereof. For instance, when the hydroxyl functionality is present in the terminal groups of the main chain, the wax may be referred to as a difunctional hydroxyl modified silicone wax and when the hydroxyl functionality is present in the side/pendant groups of the main chain, the wax may be referred to as a multi-functional hydroxyl modified silicone wax. In general, as an example utilizing polydimethylsiloxane, one or more methyl groups of a polydimethylsiloxane may be modified such that the polydimethylsiloxane comprises a hydroxyl group. However, it should be understood that, although polydimethylsiloxane is exemplified, the modifications may be made to any silicone wax known in the art.

The hydroxyl modified silicone wax may have a hydroxyl value of from at least 5, such as at least 10, such as at least 25, such as at least 55, such as at least 100, such as at least 150 to less than about 400, such as less than about 250, such as less than about 200, such as less than about 130, such as less than about 100, such as less than about 70, such as less than about 30, such as less than about 20. In general, the hydroxyl value is the measure of free hydroxyl groups in a compound as determined according to ASTM D1957.

The hydroxyl modified silicone wax may have a viscosity at 25° C. of less than about 10,000 cps, such as less than about 5,000 cps, such as less than about 2,000 cps, such as less than about 1,000 cps, such as less than about 500 cps, such as less than about 250 cps, such as less than about 100 cps and generally greater than about 1 cps, such as greater than about 50 cps, such as greater than about 100 cps, such as greater than about 1,000 cps. The viscosity may be measured according to ASTM D7042.

In another embodiment, the silicone wax may be an alkyl modified silicone wax or an alkylated silicone wax such that the silicone wax has alkyl functionality comprising an alkyl having two or more carbon atoms. For instance, the alkyl modified silicone wax may comprise alkyl side/pendant groups. In general, as an example utilizing polydimethylsiloxane, one or more methyl groups of a polydimethylsiloxane may be modified such that the polydimethylsiloxane comprises an alkyl group having 2 or more carbon atoms. However, it should be understood that, although polydimethylsiloxane is exemplified, the modifications may be made to any silicone wax known in the art.

In one embodiment, the alkyl modified silicone wax may have a melting temperature of at least about 25° C., such as at least about 30° C., such as at least about 50° C., such as at least about 60° C., such as at least about 65° C. and generally less than about 90° C., such as less than about 80° C., such as less than about 70° C., such as less than about 55° C., such as less than about 45° C., such as less than about 35° C.

In one embodiment, when the groups of the unmodified silicone wax are substituted with alkyl groups having more than 2 carbon atoms, at least about 10%, such as at least about 25%, such as at least about 40%, such as at least about 55%, such as at least about 75% alkyl groups and generally less than about 90%, such as less than about 85%, such as less than about 65%, such as less than about 45%, such as less than about 35% alkyl groups of the unmodified silicone wax may be modified to comprise alkyl groups having 2 or more carbon atoms such as from 2 to 40 carbons. These alkyl groups may be pendant groups or terminal groups.

As indicated above, the alkyl groups comprise alkyls having two or more carbon atoms. In one embodiment, the alkyl modified silicone wax may be comprised of at least one C₂₋₄₀ alkyl group, such as a C₈₋₄₀ alkyl group, such as a C₁₀₋₄₀ alkyl group, such as a C₂₀₋₄₀ alkyl group, such as a C₂₀₋₃₀ alkyl group or a C₂₅₋₃₅ alkyl group, such as a C₂₆ or a C₃₂ alkyl group.

In another embodiment, the silicone wax may be a polyether modified silicone wax such that the silicone wax has polyether functionality. The polyether functionality may be present in the side/pendant groups, terminal groups, or a combination thereof. For instance, when the polyether functionality is present in the terminal groups of the main chain, the wax may be referred to as a difunctional polyether modified silicone wax and when the polyether functionality is present in the side/pendant groups of the main chain, the wax may be referred to as a multi-functional polyether modified silicone wax. In general, as an example utilizing polydimethylsiloxane, one or more methyl groups of a polydimethylsiloxane may be modified such that the polydimethylsiloxane comprises a polyether group. However, it should be understood that, although polydimethylsiloxane is exemplified, the modifications may be made to any silicone wax known in the art.

For instance, the polyether groups may be derived from polyalkylene glycols such as polyethylene glycol, polypropylene glycol, and the like. However, it should be understood that any polyalkylene glycol or polyalkylene oxide known in the art may be utilized. For instance, the polyether group may be comprised of from 2 to 10 carbons, such as from 2 to 8 carbons, such as from 2 to 5 carbons. In addition, the polyether groups may also be linear polyether groups or branched polyether groups.

When utilizing a polydimethylsiloxane as the base silicone wax, the polyether modified silicone wax may be referred to as a dimethicone copolyol comprising silicone and polyether group(s). As provided above, in one embodiment, the polyether modified silicone wax may be polyether terminated.

In another embodiment, the silicone wax may be an amine modified silicone wax such that the silicone wax has amine functionality. The amine functionality may include primary amines, secondary amines, and/or tertiary amines. For instance, the silicone wax may be a primary amine modified, secondary amine modified, or tertiary amine modified silicone wax. For instance, when the amine functionality is present in the terminal groups of the main chain, the wax may be referred to as a difunctional amine modified silicone wax and when the amine functionality is present in the side/pendant groups of the main chain, the wax may be referred to as a multi-functional amine modified silicone wax. In general, as an example utilizing polydimethylsiloxane one or more methyl groups of a polydimethylsiloxane may be modified such that the polydimethylsiloxane comprises an amine group. However, it should be understood that, although polydimethylsiloxane is exemplified, the modifications may be made to any silicone wax known in the art.

For instance, in one embodiment, the amine group may include a dialkylamino group. For instance, the group comprising amino functionality may be comprised of two alkyl groups wherein both alkyl groups are the same or different. For instance, the alkyl groups may include any number of carbons from 2 to 30 carbons. These include, but are not limited to, ethyl, propyl, pentyl hexyl, heptyl, octyl, nonyl, decyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl, and the like. For instance, in one embodiment, the group comprising amino functionality may be a stearyl aminopropyl group, also known as an octadecyl aminopropyl group. In another embodiment, the amine group may be comprised of an aminoethylaminopropyl group. However, it should be understood that any amine group comprising amine functionality may be present in the silicone wax.

In one embodiment, the silicone wax and/or modified silicone wax identified above may have a molecular weight of at least about 100 g/mol, such as at least about 500 g/mol, such as at least about 1,000 g/mol, such as at least about 2,500 g/mol, such as at least about 5,000 g/mol, such as at least about 7,500 g/mol, such as at least about 10,000 g/mol and generally less than 100,000 g/mol, such as less than 75,000 g/mol, such as less than 50,000 g/mol, such as less than 40,000 g/mol, such as less than 30,000 g/mol, such as less than 20,000 g/mol, such as less than 18,000 g/mol, such as less than 15,000 g/mol, such as less than 10,000 g/mol, such as less than 7,500 g/mol

In one embodiment, the silicone wax and/or modified silicone wax identified above may have a viscosity at 25° C. of less than 100,000 cps, such as less than 50,000 cps, such as less than 25,000 cps, such as less than 10,000 cps, such as less than 1,000 cps, such as less than 500 cps, such as less than 250 cps, such as less than 100 cps and greater than about 1 cps, such as greater than about 10 cps, such as greater than about 100 cps, such as greater than about 500 cps, such as greater than about 1,000 cps, such as greater than about 5,000 cps.

The modified silicone wax may be in the form of a hard wax or a soft wax. In general, a hard wax may have a narrow melting point. A hard wax may be in the form of a hard solid below the melting point and a liquid above the melting point. In general, a soft wax may have a broader melting point. A soft wax may be soft below the melting point and more viscous above the melting point, in comparison to hard waxes.

In one embodiment, more than one modified silicone wax may be used as the tribological modifier. For instance, any combination of modified silicone waxes may be used according to the present invention. In another embodiment, only one modified silicone wax may be utilized with the composition of the present invention. In another embodiment, a modified silicone wax may comprise at least two different functionalities. In addition, the composition may also include an unmodified silicone wax in conjunction with a modified silicone wax.

The silicone wax and/or modified silicone wax may be present in the composition in an amount of greater than 0 wt. %, such as greater than about 0.1 wt. %, such as greater than about 0.5 wt. %, such as greater than about 0.75 wt. %, such as greater than about 1 wt. %, such as greater than about 1.5 wt. %, such as greater than about 1.75 wt. % and generally less than about 10 wt. %, such as less than about 5 wt. %, such as less than about 3 wt. %, such as less than about 2.5 wt. %, such as less than about 2.25 wt. %, wherein the weight is based on the total weight of the composition.

According to the present disclosure, various other tribological modifiers may be incorporated into the composition. These tribological modifiers may include, for instance, calcium carbonate particles, ultrahigh-molecular-weight polyethylene (UHMW-PE) particles, stearyl stearate particles, silicone oil, UHMW-Si, a polyethylene wax, an amide wax, wax particles comprising an aliphatic ester wax comprised of a fatty acid and a monohydric alcohol, a graft copolymer with an olefin polymer as a graft base, or a combination thereof. These tribological modifiers include the following:

(1) From 0.1-50 wt. %, such as from 1-25 wt. %, of a calcium carbonate particle such as a calcium carbonate (chalk) powder.

(2) From 0.1-50 wt. %, such as from 1-25 wt. %, such as from 2.5-20 wt. %, such as from 5 to 15 wt. %, of an ultrahigh-molecular-weight polyethylene (UHMW-PE) powder. UHMW-PE can be employed as a powder, in particular as a micro-powder. The UHMW-PE generally has a mean particle diameter D₅₀ (volume based and determined by light scattering) in the range of 1 to 5000 μm, preferably from 10 to 500 μm, and particularly preferably from 10 to 150 μm such as from 30 to 130 μm, such as from 80 to 150 μm, such as from 30 to 90 μm.

The UHMW-PE can have an average molecular weight of higher than 1.0·10⁶ g/mol, such as higher than 2.0·10⁶ g/mol, such as higher than 4.0·10⁶ g/mol, such as ranging from 1.0·10⁶ g/mol to 15.0·10⁶ g/mol, such as from 3.0·10⁶ g/mol to 12.0·10⁶ g/mol, determined by viscosimetry. Preferably, the viscosity number of the UHMW-PE is higher than 1000 ml/g, such as higher than 1500 ml/g, such as ranging from 1800 ml/g to 5000 ml/g, such as ranging from 2000 ml/g to 4300 ml/g (determined according to ISO 1628, part 3; concentration in decahydronaphthalin: 0.0002 g/ml).

(3) From 0.1-10 wt. %, such as from 0.1-5 wt. %, such as from 0.5-3 wt. %, of stearyl stearate.

(4) From 0.1-10 wt. %, such as from 0.5-5 wt. %, such as from 0.8-2 wt. %, of a silicone oil. Alternatively, in one embodiment, the composition may be substantially free of silicone oil, such as less than about 0.1 wt. %, such as less than about 0.05 wt. %, such as less than about 0.01 wt. %, such as about 0 wt. %.

When silicone oil is present in the composition, the silicone oil may have an average molecular weight of at least about 5,000 g/mol, such as at least about 20,000 g/mol, such as at least about 50,000 g/mol and generally less than 100,000 g/mol, such as less than about 75,000 g/mol, such as less than about 50,000 g/mol. The silicone oil may have a kinematic viscosity at 40° C. measured according to DIN 51562 of greater than about 100 mm² s⁻¹, such as greater than about 5,000 mm² s⁻¹, such as greater than about 15,000 mm² s⁻¹ and generally less than about 100,000 mm² s⁻¹, such as less than about 50,000 mm² s¹, such as less than about 25,000 mm² s⁻¹, such as less than about 15,000 mm² s⁻¹. The silicone oil may comprise a liquid polysiloxane such as a polydimethylsiloxane at a room temperature of 25° C. with the above molecular weight and/or kinematic viscosity specifications.

(5) From 0.1-10 wt. %, such as from 0.5-5 wt. %, such as from 0.8-2 wt. %, of a silicone oil. The UHMW-Si may have an average molecular weight of greater than about 300,000 g/mol, such as greater than about 500,000 g/mol, such as greater than about 1,000,000 g/mol and less than about 10,000,000 g/mol.

(6) From 0.1-5 wt. %, such as from 0.5-3 wt. %, of a polyethylene wax, such as an oxidized polyethylene wax.

(7) From 0.1-5 wt. %, such as from 0.2-2 wt. %, of an amide wax.

(8) From 0.1-5 wt. %, such as from 0.5-3 wt. %, of an aliphatic ester wax composed of a fatty acid and of a monohydric alcohol.

(9) From 0.1-50 wt. %, such as from 1-25 wt. %, such as from 2-10 wt. % by weight of a graft copolymer which has an olefin polymer as a graft base and, grafted on this, at least one vinyl polymer or one ether polymer, and/or a graft copolymer which has an elastomeric core based on polydienes and a hard graft envelope composed of (meth)acrylates and/or of (meth)acrylonitriles. A suitable graft base can be any olefin homopolymer (e.g., polyethylene or polypropylene) or copolymer or copolymers derived from copolymerizable ethylenically unsaturated monomers (e.g, ethylenepropylene copolymers, ethylene-1-butene copolymers, ethylene/glycidyl (meth)acrylate copolymers). Suitable graft monomers are any of the ethylenically unsaturated monomers having a polar group or other graftable monomers having polar groups that modify the polarity of the essentially non-polar graft base (e.g. ethylenically unsaturated carboxylic acids such as (meth)acrylic acid and derivatives thereof in combination with acrylonitrile or styrene/acrylonitrile, if appropriate). In one embodiment, the graft copolymer may comprise a polyethylene or polypropylene graft base grafted with acrylonitrile or with styrene/acrylonitrile.

The tribological modifiers and in particular the silicone wax such as the modified silicone wax identified herein may improve the surface and physical properties of the compositions such as the tribological properties. For instance, it may provide a composition and a polymer article with better slip, surface smoothness, flexibility, excellent lubricity and gloss. In addition, the tribological modifiers improve the tribological properties of the composition by reducing the coefficient of friction and wear when contacted with another surface or counter-material. In addition, the tribological properties may even be improved without the need for an external lubricant, such as water-based or PTFE-based external lubricants.

Other Additives

The polyoxymethylene polymer composition and polymer article produced therefrom may also contain other known additives such as, for example, antioxidants, formaldehyde scavengers, acid scavengers, UV stabilizers or heat stabilizers, reinforcing fibers. In addition, the compositions can contain processing auxiliaries, for example adhesion promoters, lubricants, nucleating agents, demolding agents, fillers, or antistatic agents and additives which impart a desired property to the compositions and articles produced therefrom. The sum of the wt. % of all of the components utilized in the polyoxymethylene polymer composition will be 100 wt. %

In one embodiment, an ultraviolet light stabilizer may be present. The ultraviolet light stabilizer may comprise a benzophenone, a benzotriazole, a benzoate, or a combination thereof. The UV light absorber, when present, may be present in the composition in an amount of at least about 0.01 wt. %, such as at least about 0.05 wt. %, such as at least about 0.1 wt. % and less than about 1 wt. %, such as less than about 0.5 wt. %, such as less than about 0.25 wt. %, wherein the weight is based on the total weight of the composition.

In one embodiment, a formaldehyde scavenger, such as a nitrogen-containing compound, may be present. Mainly, of these are heterocyclic compounds having at least one nitrogen atom as a hetero atom which is either adjacent to an amino-substituted carbon atom or to a carbonyl group, for example pyridine, pyrimidine, pyrazine, pyrrolidone, aminopyridine and compounds derived therefrom. Other particularly advantageous compounds are triamino-1,3,5-triazine (melamine) and its derivatives, such as melamine-formaldehyde condensates and methylol melamine. Polyamides such as copolyamides and/or oligomeric polyamides are also suitable in principle for use as formaldehyde scavengers. Further, the formaldehyde scavenger may be a guanamine compound which may include an aliphatic guanamine-based compound, an alicyclic guanamine-based compound, an aromatic guanamine-based compound, a hetero atom-containing guanamine-based compound, or the like. The formaldehyde scavengers may be used individually or in combination. The formaldehyde scavenger may be present in the composition in an amount of at least about 0.001 wt. %, such as at least about 0.005 wt. %, such as at least about 0.01 wt. %, such as at least about 0.025 wt. % and less than about 1 wt. %, such as less than about 0.50 wt. %, such as less than about 0.25 wt. %, such as less than about 0.10 wt. %, such as less than about 0.075 wt. %, wherein the weight is based on the total weight of the composition.

In one embodiment, an acid scavenger may be present. The acid scavenger may comprise, for instance, an alkaline earth metal salt. For instance, the alkaline earth metal salt may comprise a calcium salt, such as a calcium citrate or tricalcium citrate. The acid scavenger may be present in an amount of at least about 0.001 wt. %, such as at least about 0.005 wt. %, such as at least about 0.010 wt. %, such as at least about 0.025 wt. % and less than about 1 wt. %, such as less than about 0.50 wt. %, such as less than about 0.25 wt. %, such as less than about 0.10 wt. %, such as less than about 0.075 wt. %, wherein the weight is based on the total weight of the composition.

In one embodiment, a nucleating agent may be present. The nucleating agent may increase crystallinity and may comprise a polyoxymethylene terpolymer. In one embodiment, for instance, the nucleating agent may comprise a terpolymer of butanediol diglycidyl ether, ethylene oxide, and trioxane. The nucleating agent may be present in the composition in an amount of at least about 0.01 wt. %, such as at least about 0.05 wt. %, such as at least about 0.1 wt. %, such as at least about 0.2 wt. % and less than about 2 wt. %, such as less than about 1 wt. %, such as less than about 0.5 wt. %, such as less than about 0.3 wt. %, wherein the weight is based on the total weight of the composition.

In one embodiment, an antioxidant, such as a sterically hindered phenol, may be present. The antioxidant may comprise pentaerythrityl tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], triethylene glycol bis[3-(3-tert-butyl-4-hydroxy-5-methylphenyl)propionate], 3,3′-bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionohydrazide], and hexamethylene glycol bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate]. The antioxidant may be present in the composition in an amount of at least about 0.01 wt. %, such as at least about 0.05 wt. %, such as at least about 0.10 wt. %, such as at least about 0.15 wt. % and less than about 1 wt. %, such as less than about 0.5 wt. %, such as less than about 0.25 wt. % wherein the weight is based on the total weight of the composition.

In one embodiment, a light stabilizer, such as a sterically hindered amine, may be present in lieu of or in addition to the ultraviolet light stabilizer. Hindered amine light stabilizers that may be used include oligomeric hindered amine compounds that are N-methylated. For instance, hindered amine light stabilizer may comprise a high molecular weight hindered amine stabilizer. The light stabilizers, when present, may be present in the composition in an amount of at least about 0.01 wt. %, such as at least about 0.05 wt. %, such as at least about 0.075 wt. % and less than about 1 wt. %, such as less than about 0.75 wt. %, such as less than about 0.5 wt. %, wherein the weight is based on the total weight of the composition.

In one embodiment, a lubricant may be present. The lubricant may comprise a polymer wax composition. In one embodiment, a polyethylene glycol polymer (processing aid) may be present in the composition. The polyethylene glycol, for instance, may have a molecular weight of from about 1000 to about 5000, such as from about 3000 to about 4000. In one embodiment, for instance, PEG-75 may be present. In another embodiment, a fatty acid amide such as ethylene bis(stearamide) may be present. Lubricants may generally be present in the polymer composition in an amount of at least about 0.05 wt. %, such as at least about 0.10 wt. %, such as at least about 0.15 wt. % and less than about 1 wt. %, such as less than about 0.50 wt. %, such as less than about 0.25 wt. %, wherein the weight is based on the total weight of the composition.

In one embodiment, a compatibilizer, such as a phenoxy resin, may be present. Generally, the phenoxy resin may be present in the composition in an amount of at least about 0.01 wt. %, such as at least about 0.05 wt. %, such as at least about 0.1 wt. % and less than about 1 wt. %, such as less than about 0.5 wt. %, such as less than about 0.25 wt. %, wherein the weight is based on the total weight of the composition.

In one embodiment, a colorant may be present. Colorants that may be used include any desired inorganic pigments, such as titanium dioxide, ultramarine blue, cobalt blue, and other organic pigments and dyes, such as phthalocyanines, anthraquinnones, and the like. Other colorants include carbon black or various other polymer-soluble dyes. The colorant may be present in the composition in an amount of at least about 0.01 wt. %, such as at least about 0.05 wt. %, such as at least about 0.1 wt. % and less than about 5 wt. %, such as less than about 2.5 wt. %, such as less than about 1 wt. %, wherein the weight is based on the total weight of the composition.

In one embodiment, a coupling agent may be present. Coupling agents used may include polyfunctional coupling agents, such as trifunctional or bifunctional agents. A suitable coupling agent may be a polyisocyanate such as a diisocyanate. The coupling agent may provide a linkage between the polyoxymethylene polymer and, for example, a reinforcing fiber and/or sizing material coated on a reinforcing fiber. Generally, when present, the coupling agent is present in an amount of at least about 0.1 wt. %, such as at least about 0.2 wt. % such as at least about 0.3 wt. % and less than about 5 wt. %, such as less than about 3 wt. %, such as less than about 1 wt. %. Alternatively, the composition may also be substantially free of any coupling.

In one embodiment, a reinforcing fiber may be present. The reinforcing fibers which may be used according to the present invention include mineral fibers, glass fibers; polymer fibers such as aramid fibers, metal fibers such as steel fibers, carbon fibers, or natural fibers. These fibers may be unmodified or modified, e.g. provided with a sizing or chemically treated, in order to improve adhesion to the polymer. Fiber diameters can vary depending upon the particular fiber used and whether the fiber is in either a chopped or a continuous form. The fibers, for instance, can have a diameter of from about 5 μm to about 100 μm, such as from about 5 μm to about 50 μm, such as from about 5 μm to about 15 μm. When present, the composition may contain reinforcing fibers in an amount of at least 1 wt. %, such as at least 5 wt. %, such as at least 7 wt. %, such as at least 10 wt. %, such as at least 15 wt. % and generally less than about 50 wt. %, such as less than about 45 wt. %, such as less than about 40 wt. %, such as less than about 30 wt. %, such as less than about 20 wt. %, wherein the weight is based on the total weight of the composition. Alternatively, the composition may also be substantially free, such as less than about 0.1 wt. %, such as less than about 0.05 wt. %, such as less than about 0.01 wt. %, such as about 0 wt. %, of any reinforcing fibers.

In addition, in one embodiment, the composition of the present disclosure may be substantially free of an olefin polymer and/or copolymer such as polyethylene, polypropylene, and the like. In another embodiment, the copolymer of the present disclosure may be substantially free of a carboxylic hydrazide. As used herein, substantially free refers to less than about 0.1 wt. %, such as less than about 0.05 wt. %, such as less than about 0.01 wt. %, such as about 0 wt. %.

Polymer Articles

The compositions of the present disclosure can be compounded and formed into a polymer article using any technique known in the art. For instance, the composition can be intensively mixed to form a substantially homogeneous blend. The blend can be melt kneaded at an elevated temperature, such as a temperature that is higher than the melting point of the polymer utilized in the polymer composition but lower than the degradation temperature. Alternatively, the composition can be melted and mixed together in a conventional single or twin screw extruder. Preferably, the melt mixing is carried out at a temperature ranging from 100 to 280° C., such as from 120 to 260° C., such as from 140 to 240° C., such as from 180 to 220° C.

After extrusion, the composition may be formed into pellets. The pellets can be molded into polymer articles by techniques known in the art such as injection molding, thermoforming, blow molding, rotational molding and the like. According to the present disclosure, the polymer articles demonstrate excellent tribological behavior and mechanical properties. Consequently, the polymer articles can be used for several applications where low wear and excellent tribological and mechanical properties are desired.

Polymer articles include any moving articles or moldings that are in contact with another surface and may require high tribological requirements. For instance, polymer articles include articles for the automotive industry, especially housings, latches such as rotary latches, window winding systems, wiper systems, pulleys, sun roof systems, seat adjustments, levers, bushes, gears, gear boxes, claws, pivot housings, wiper arms, brackets or seat rail bearings, zippers, switches, cams, rollers or rolling guides, sliding elements or glides such as sliding plates, conveyor belt parts such as chain elements and links, castors, fasteners, levers, conveyor system wear strips and guard rails, medical equipment such as medical inhalers and injectors. An almost limitless variety of polymer articles may be formed from the polymer compositions of the present disclosure.

Properties

Utilizing the polyoxymethylene polymer composition and polymer article produced therefrom according to the present disclosure provides compositions and articles with improved tribological properties and low wear. According to the present disclosure, the tribological properties are generally measured by the coefficient of friction and wear.

In general, static friction is the friction between two or more surfaces that are not moving relative to each other (ie., both objects are stationary). In general, dynamic friction occurs when two objects are moving relative to each other (ie., at least one object is in motion or repeated back and forth motion). In general, wear refers to the removal of material from or the impairment of a solid surface resulting from friction or impact. As such, when surfaces contact one another, they not only experience friction but they also experience wear.

According to the present disclosure, the composition and polymer article may exhibit a static coefficient of friction against another surface, such as a polyacetal or polyoxymethylene surface, as determined according to VDA 230-206 utilizing a ball-on-plate configuration with a load of 30N, sliding speed of 8 mm/s, and a test duration of 45 minutes, of greater than about 0.05, such as greater than about 0.10, such as greater than about 0.20, such as greater than about 0.25, such as greater than about 0.30, such as greater than about 0.35 and generally less than about 0.75, such as less than about 0.50, such as less than about 0.45, such as less than about 0.40, such as less than about 0.38, such as less than about 0.35. The above values may be obtained when tested against an unmodified polyoxymethylene surface such as a tribologically unmodified polyoxymethylene surface.

According to the present disclosure, the composition and polymer article may exhibit a dynamic coefficient of friction against another surface, such as a polyacetal or polyoxymethylene surface, as determined according to VDA 230-206 utilizing a ball-on-plate configuration with a load of 30N, sliding speed of 8 mm/s, and a test duration of 45 minutes, of greater than about 0.05, such as greater than about 0.10, such as greater than about 0.15, such as greater than about 0.20, and generally less than about 0.50, such as less than about 0.40, such as less than about 0.35, such as less than about 0.30, such as less than about 0.26. The above values may be obtained when tested against an unmodified polyoxymethylene surface such as a tribologically unmodified polyoxymethylene surface.

According to the present disclosure, the wear is also measured as a measure of the width of the wear track according to VDA 230-206. Generally, before the test begins, there is no wear track as there is no wear or loss of material. However, after the test is completed, generally, one may observe wear track due to the loss of material. Generally, the width of the track provides a measure of the wear. According to the present disclosure, the composition and polymer article may also exhibit a wear track width of less than 2 mm, such as less than about 1.95 mm, such as less than about 1.75 mm, such as less than about 1.6 mm, such as less than about 1.5 mm and generally greater than 0 mm, such as greater than about 0.5 mm, such as greater than about 1 mm, such as greater than about 1.25 mm. In comparison, generally, tribologically unmodified polyoxymethylene compositions may exhibit a wear track width of at least 2 mm or greater.

In addition, the composition and polymer article of the present disclosure may be utilized in high speed applications, such as in conveyor applications. Generally, when utilized in high speed applications, compositions and articles may exhibit substantial wear resulting in poor performance and requiring frequent replacement of parts. However, the present inventors have discovered that the composition of the present disclosure exhibits significantly improved wear properties, especially when operating at high speeds.

For instance, in one embodiment, the composition and articles of the present disclosure may exhibit improved wear properties when compared to polyoxymethylene compositions and articles that are not modified with a tribological modifier. The abrasive wear tests may be conducted utilizing a block on shaft test. For instance, the tests utilize an abrasion shaft and cylindrical test specimens of 12 mm diameter comprising the specimen or material to be tested. The shaft may be comprised of steel and have a diameter of 65 mm and the test conditions include a load of 3.1 N, a sliding velocity of 136 m/min, a roughness of about 0.8 μm, a test duration of 60 h, and a distance of 490 km.

The relative wear volume or volumetric wear or loss of the specimens may be calculated relative to the wear volume or volumetric loss/wear of a tribologically unmodified polyoxymethylene. The relative wear volume or volumetric loss % of the specimens can be determined according to the equation: Relative volumetric loss (%)=(Wear Volume of Specimen/Wear Volume of Tribologically Unmodified Polyoxymethylene)*00%. For instance, the volumetric loss or wear may be reduced by at least 50%, such as at least 75%, such as at least 90%, such as at least about 95%, such as at least about 97%, such as at least about 98%, such as at least about 99% and less than about 100%, such as less than about 99%, such as less than about 98%, when compared to the volumetric loss or wear volume of a polyoxymethylene polymer that is not modified with a tribological modifier. Accordingly, the composition of the present disclosure exhibits significantly improved wear properties, especially when utilized at high sliding speeds.

As just one example, the relative volumetric loss of a polyoxymethylene polymer that is not modified with a tribological modifier is 100%. As an example, if the composition and/or article of the present disclosure exhibits a relative volumetric loss of 5%, the polyoxymethylene polymer composition and/or article of the present disclosure exhibits a volumetric wear reduced by approximately 95%, in comparison to the unmodified polyoxymethylene.

An unmodified polyoxymethylene polymer composition or tribologically unmodified polyoxymethylene polymer composition may be substantially free of a tribological modifier such that it contains less than about 0.1 wt. %, such as less than about 0.05 wt. %, such as about 0 wt. % of a tribological modifier.

In one embodiment, the above static coefficient of friction and dynamic coefficient of friction values and wear properties are exhibited between the composition or polymer article and various counter-materials. For instance, the above values may be exhibited between the composition or polymer article and a polyester surface such as a polyethylene terephthalate surface. In another embodiment, the above values may be exhibited between the composition or polymer article and a polyacetal surface, a metal surface such as a steel surface, or a polyolefin surface such as a polypropylene surface or a polyethylene surface such as an ultra-high molecular weight polyethylene surface.

The present disclosure may be better understood with reference to the following examples.

EXAMPLES

The examples of the invention are given below by way of illustration and not by way of limitation. The following experiments were conducted in order to show some of the benefits and advantages of the present invention.

Example 1

The polyoxymethylene polymer compositions comprising a polyoxymethylene polymer and a silicone wax such as a modified silicone wax were produced in accordance with the present disclosure. The relative amount of each component of the composition is provided in Table 1. The polyoxymethylene copolymer was comprised of a copolymer made from trioxane and dioxolane with a melt volume flow rate of 8 cm³/10 min as determined according to ISO 1133.

Comparative Sample 1 utilized a polyoxymethylene polymer without tribological modifiers. Samples 1-7 were comprised of polyoxymethylene and a silicone wax such as a modified silicone wax. As shown in Table 1, seven silicone waxes were utilized to tribologically modify the polyoxymethylene polymer. These waxes are identified as:

-   -   Silicone Wax 1—alkyl modified silicone wax comprising 80% alkyl         pendant groups, T_(m)=65° C.     -   Silicone Wax 2—alkyl modified silicone wax comprising 35% alkyl         pendant groups, T_(m)=60° C.     -   Silicone Wax 3—alkyl modified silicone wax, T_(m)=43° C.,         MW=12,600 g/mol     -   Silicone Wax 4—amine modified silicone wax, T_(m)=39° C.,         MW=13,100 g/mol     -   Silicone Wax 5—polyether terminated modified silicone wax,         MW—6,200 g/mol     -   Silicone Wax 6—hydroxy modified silicone wax, MW—8,800 g/mol     -   Silicone Wax 7—alkyl modified silicone wax, T_(m)=25-40° C.,         MW=6,000 g/mol

The components of each composition were mixed together and compounded using a ZSK 25MC (Werner & Pfleiderer, Germany) twin screw extruder (zone temperature 190° C., melt temperature about 210° C.). The screw configuration with kneading elements was chosen so that effective and thorough mixing of the components took place. The compositions were extruded and pelletized. The pellets were dried for 8 hours at 120° C. and then injection molded. The process conditions were selected in accordance with recommendations of ISO-9988-2, material standard for polyoxymethylene.

The tribological properties (dynamic and static coefficient of friction) were determined between the polyoxymethylene composition and a steel surface. Stick-slip tests were conducted to determine the dynamic coefficient of friction, the static coefficient of friction, and the wear track width. Stick-slip tests were conducted according to VDA 230-206. A ball-on-plate configuration was utilized with a load of 30 N, sliding speed of 8 mm/s, a test duration of 8 minutes, and a test temperature of 23° C.

The volumetric wear loss tests were conducted utilizing a block on steel shaft test. The tests were conducted utilizing an abrasion shaft and cylindrical test specimens of 12 mm diameter made from the composition to be tested. The shaft was comprised of steel and had a diameter of 65 mm. The test conditions included a load of 3.1 N, a sliding velocity of 136 m/min, a roughness of about 0.8 μm, a test duration of 60 h, and a distance of 490 km.

The relative volumetric wear loss % reported is based relative to Comparative Sample 1. In particular, the relative volumetric wear loss % of the specimens was calculated relative to the wear volume of the tribologically unmodified polyoxymethylene. The wear volume of the tribologically unmodified polyoxymethylene was calculated to be 7.9 mm³. The relative volumetric wear loss % of the specimens was determined according to the equation: Relative volumetric wear loss (%)=(Wear Volume of Specimen/Wear Volume of Tribologically Unmodified Polyoxymethylene)*100%.

TABLE 1 Comparative Sample Sample Sample Sample Sample Sample Sample Sample 1 1 2 3 4 5 6 7 Polyoxymethylene 99.25 97.25 97.25 97.25 97.25 97.25 97.25 97.25 copolymer (wt. %) Polyoxymethylene 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 terpolymer (wt. %) Silicone wax 1 — 2.00 — — — — — — (wt. %) Silicone wax 2 — — 2.00 — — — — — (wt. %) Silicone wax 3 — — — 2.00 — — — — (wt. %) Silicone wax 4 — — — — 2.00 — — — (wt. %) Silicone wax 5 — — — — — 2.00 — — (wt. %) Silicone wax 6 — — — — — — 2.00 — (wt. %) Silicone wax 7 — — — — — — — 2.00 (wt. %) Triethylene glycol 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 bis[3-(3-tert-butyl- 4-hydroxy-5- methylphenyl) propionate] (wt. %) Ethylene 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 bisstearamide (wt. %) Tricalcium citrate 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 (wt. %) Polyamide (wt. %) 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 Stick-Slip Test Dynamic CoF 0.342 0.253 0.234 0.243 0.207 0.203 0.249 0.252 Static CoF 0.457 0.372 0.367 0.301 0.323 0.384 0.372 0.367 Wear track width 2.00 1.91 1.47 1.91 1.50 1.57 1.59 1.63 (mm) Block on Steel Shaft Test Relative 100 0.7 1.4 1.4 1.6 3.9 4.4 4.4 volumetric wear loss (%)

Based on the above, the relative volumetric wear loss of Sample 1 was 0.7% of the loss of Comparative Sample 1. Likewise, for samples 2-7, the loss was 1.4%, 1,4%, 1.6%, 3.9%, 4.4%, and 4.4%, respectively, of the loss of Comparative Sample 1. Accordingly, the volumetric wear loss was reduced by 99.3% utilizing the composition of Sample 1 when compared to Comparative Sample 1. Similarly, the reduction is by 98.6%, 98.6%, 98.4%, 96.1%, 95.6%, and 95.6%, respectively, for Samples 2-7.

As shown in the examples, the composition of the present invention exhibits improved tribological properties in comparison to tribologically unmodified polyoxymethylene. In particular, the composition of the present invention exhibits significant improvements in the wear performance of the composition and articles produced therefrom at high sliding speeds.

These and other modifications and variations to the present invention may be practiced by those of ordinary skill in the art, without departing from the spirit and scope of the present invention, which is more particularly set forth in the appended claims. In addition, it should be understood that aspects of the various embodiments may be interchanged both in whole or in part.

Furthermore, those of ordinary skill in the art will appreciate that the foregoing description is by way of example only, and is not intended to limit the invention so further described in such appended claims. 

1. A polyoxymethylene polymer composition comprising a polyoxymethylene copolymer, and at least one silicone wax having a molecular weight of less than 40,000 g/mol.
 2. The polymer composition of claim 1, wherein the silicone wax comprises a modified silicone wax.
 3. The polymer composition of claim 1, wherein the silicone wax has a molecular weight of less than about 30,000 g/mol.
 4. The polymer composition of claim 1, wherein the silicone wax comprises a modified polydimethylsiloxane wax.
 5. The polymer composition of claim 1, wherein the silicone wax comprises an alkyl group having more than 2 carbon atoms, an amine group, a polyether group, a hydroxyl group, or any combination thereof.
 6. The polymer composition of claim 5, wherein the silicone wax is comprised of pendant groups and wherein from about 10% to about 90% of the pendant groups are comprised of the alkyl group having more than 2 carbon atoms.
 7. The polymer composition of claim 5, wherein the silicone wax comprises a polyether group and wherein the polyether group comprises from 2 to 5 carbon atoms.
 8. The polymer composition of claim 5, wherein the silicone wax comprises a hydroxyl group and wherein the hydroxyl group has a hydroxyl value of from 5 to
 150. 9. The polymer composition of claim 1, wherein the silicone wax is present in an amount of from about 0.1 wt. % to about 10 wt. %.
 10. The polymer composition of claim 1, wherein the composition further comprises a nucleating agent in an amount of from about 0.05 wt. % to about 0.5 wt. %.
 11. The polymer composition of claim 10, wherein the nucleating agent is a polyoxymethylene terpolymer.
 12. The polymer composition of claim 1, wherein the composition further comprises an antioxidant in an amount of from about 0.05 wt % to about 0.5 wt. %.
 13. The polymer composition of claim 1, wherein the composition further comprises a formaldehyde scavenger in an amount of from about 0.005 wt. % to about 0.1 wt. %.
 14. The polymer composition of claim 1, wherein the composition further comprises an acid scavenger in an amount of from about 0.005 wt. % to about 0.1 wt. %.
 15. The polymer composition of claim 1, wherein the composition further comprises a lubricant in an amount of from about 0.05 wt. % to about 0.5 wt. %.
 16. The polymer composition of claim 1, wherein the composition has a dynamic coefficient of friction against a polyacetal surface of from about 0.1 to about 0.5.
 17. A polymer article comprising the composition of claim
 1. 18. A polymer article comprising a polyoxymethylene polymer composition comprising: a polyoxymethylene copolymer, at least one silicone wax having a molecular weight of less than 40,000 g/mol, and at least one of an antioxidant, a formaldehyde scavenger, a nucleating agent, an acid scavenger, a lubricant, or any combination thereof.
 19. The polymer article of claim 18, wherein the silicone wax comprises a polydimethylsiloxane wax.
 20. The polymer article of claim 18, wherein the silicone wax has a molecular weight of less than about 30,000 g/mol.
 21. The polymer article of claim 18, wherein the silicone wax comprises an alkyl group having more than 2 carbon atoms, an amine group, a polyether group, a hydroxyl group, or any combination thereof.
 22. The polymer article of claim 21, wherein the silicone wax is comprised of pendant groups and wherein from about 10% to about 90% of the pendant groups are comprised of the alkyl group having more than 2 carbon atoms.
 23. The polymer article of claim 21, wherein the silicone wax comprises a polyether group and wherein the polyether group comprises from 2 to 5 carbon atoms.
 24. The polymer article of claim 21, wherein the silicone wax comprises a hydroxyl group and wherein the hydroxyl group has a hydroxyl value of from 5 to
 150. 25. The polymer article of claim 18, wherein the composition comprises a polyoxymethylene terpolymer in an amount of from about 0.05 wt. % to about 0.5 wt. %.
 26. The polymer article of claim 18, wherein the composition comprises an antioxidant in an amount of from about in an amount of from about 0.05 wt % to about 0.5 wt. %, a formaldehyde scavenger in an amount of from about 0.005 wt. % to about 0.1 wt. %, an acid scavenger in an amount of from about 0.005 wt. % to about 0.1 wt. %, a lubricant in an amount of from about 0.05 wt. % to about 0.5 wt. %, or any combination thereof.
 27. The polymer article of claim 18, wherein the article exhibits a dynamic coefficient of friction against a polyacetal surface of from about 0.1 to about 0.5. 